1. Field of the Invention
The present invention relates generally to the field of data and voice communications over a network of nodes in a cable television plant. More specifically, it relates to the transmission of signals on the upstream path to an upstream receiver in the headend using a virtual lookahead feature.
2. Discussion of Related Art
The cable TV industry has been upgrading its signal distribution and transmission infrastructure since the late 1980s. In many cable television markets, the infrastructure and topology of cable systems now include fiber optics as part of their signal transmission components. This has accelerated the pace at which the cable industry has taken advantage of the inherent two-way communication capability of cable systems. The cable industry is now poised to develop reliable and efficient two-way transmission of digital data over its cable lines at speeds orders of magnitude faster than those available through telephone lines, thereby allowing its subscribers to access digital data for uses ranging from Internet access to cablecommuting.
Originally, cable TV lines were exclusively coaxial cable. The system included a cable head end, i.e. a distribution hub, which received analog signals for broadcast from various sources such as satellites, broadcast transmissions, or local TV studios. Coaxial cable from the head end was connected to multiple distribution nodes, each of which could supply many houses or subscribers. From the distribution nodes, trunk lines (linear sections of coaxial cable) extended toward remote sites on the cable network. A typical trunk line is about 10 kilometers. Branching off of these trunk lines were distribution or feeder cables (40% of the system's cable footage) to specific neighborhoods, and drop cables (45% of the system's cable footage) to homes receiving cable television. Amplifiers were provided to maintain signal strength at various locations along the trunk line. For example, broadband amplifiers are required about every 2000 feet depending on the bandwidth of the system. The maximum number of amplifiers that can be placed in a run or cascade is limited by the build-up of noise and distortion. This configuration, known as tree and branch, is still present in older segments of the cable TV market.
With cable television, a TV analog signal received at the head end of a particular cable system is broadcast to all subscribers on that cable system. The subscriber simply needed a television with an appropriate cable receptor to receive the cable television signal. The cable TV signal was broadcast at a radio frequency range of about 50 to 700 MHz. Broadcast signals were sent downstream; that is, from the head end of the cable system across the distribution nodes, over the trunk line, to feeder lines that led to the subscribers. However, the cable system did not have installed the equipment necessary for sending signals from subscribers to the head end, known as return or upstream signal transmission.
In the 1990s, cable companies began installing optical fibers between the head end of the cable system and distribution nodes (discussed in greater detail with respect to FIG. 1). The optical fibers reduced noise, improved speed and bandwidth, and reduced the need for amplification of signals along the cable lines. In many locations, cable companies installed optical fibers for both downstream and upstream signals. The resulting systems are known as hybrid fiber-coaxial (HFC) systems. Upstream signal transmission was made possible through the use of duplex or two-way filters. These filters allow signals of certain frequencies to go in one direction and of other frequencies to go in the opposite direction. This new upstream data transmission capability allowed cable companies to use set-top cable boxes and allowed subscribers pay-per-view functionality, i.e. a service allowing subscribers to send a signal to the cable system indicating that they want to see a certain program.
In addition, cable companies began installing fiber optic lines into the trunk lines of the cable system in the late 1980s. A typical fiber optic trunk line can be up to 80 kilometers, whereas a typical coaxial trunk line is about 10 kilometers, as mentioned above. Prior to the 1990s, cable television systems were not intended to be general-purpose communications mechanisms. Their primary purpose was transmitting a variety of entertainment television signals to subscribers. Thus, they needed to be one-way transmission paths from a central location, known as the head end, to each subscriber's home, delivering essentially the same signals to each subscriber. HFC systems run fiber deep into the cable TV network offering subscribers more neighborhood specific programming by segmenting an existing system into individual serving areas between 500 to 2,000 subscribers. Although networks using exclusively fiber optics would be optimal, presently cable networks equipped with HFC configurations are capable of delivering a variety of high bandwidth, interactive services to homes for significantly lower costs than networks using only fiber optic cables.
FIG. 1 is a block diagram of a two-way hybrid fiber-coaxial (HFC) cable system utilizing a cable modem for data transmission. It shows a head end 102 (essentially a distribution hub) which can typically service about 40,000 subscribers. Head end 102 contains a cable modem termination system (CMTS) 104 that is needed when transmitting and receiving data using cable modems. Block 104 of FIG. 1 represents a cable modem termination system connected to a fiber node 108 by pairs of optical fibers 106. Primary functions of the CMTS include (1) receiving broadband data inputs from external sources 100 and converting the data for transmission over the cable plant (e.g., converting Ethernet or ATM broadband data to data suitable for transmission over the cable system); (2) providing appropriate Media Access Control (MAC) level packet headers for data received by the cable system, and (3) modulating and demodulating the data to and from the cable system.
Head end 102 is connected through pairs of fiber optic lines 106 (one line for each direction) to a series of fiber nodes 108. Each head end can support normally up to 80 fiber nodes. Pre-HFC cable systems used coaxial cables and conventional distribution nodes. Since a single coaxial cable was capable of transmitting data in both directions, one coaxial cable ran between the head end and each distribution node. In addition, because cable modems were not used, the head end of pre-HFC cable systems did not contain a CMTS. Returning to FIG. 1, each of the fiber nodes 108 is connected by a coaxial cable 110 to two-way amplifiers or duplex filters 112 which permit certain frequencies to go in one direction and other frequencies to go in the opposite direction (frequency ranges for upstream and downstream paths are discussed below). Each fiber node 108 can normally service up to 500 subscribers. Fiber node 108, coaxial cable 110, two-way amplifiers 112, plus distribution amplifiers 114 along trunk line 116, and subscriber taps, i.e. branch lines 118, make up the coaxial distribution system of an HFC system. Subscriber tap 118 is connected to a cable modem 120. Cable modem 120 is, in turn, connected to a subscriber computer 122.
Recently, it has been contemplated that HFC cable systems could be used for two-way transmission of digital data. The data may be Internet data, digital audio, or digital video data, in MPEG format, for example, from one or more external sources 100. Using two-way HFC cable systems for transmitting digital data is attractive for a number of reasons. Most notably, they provide up to a thousand times faster transmission of digital data than is presently possible over telephone lines. However, in order for a two-way cable system to provide digital communications, subscribers must be equipped with cable modems, such as cable modem 120. With respect to Internet data, the public telephone network has been used, for the most part, to access the Internet from remote locations. Through telephone lines, data is typically transmitted at speeds ranging from 2,400 to 33,600 bits per second (bps) using commercial (and widely used) data modems for personal computers. Using a two-way HFC system as shown in FIG. 1 with cable modems, data may be transferred at speeds up to 10 million bps. Table 1 is a comparison of transmission times for transmitting a 500 kilobyte image over the Internet.
Time to Transmit a Single 500 kbyte ImageTelephone Modem (28.8 kbps)6–8 minutesISDN Line (64 kbps)1–1.5 minutesCable Modem (10 Mbps)1 second
Furthermore, subscribers can be fully connected twenty-four hours a day to services without interfering with cable television service or phone service. The cable modem, an improvement of a conventional PC data modem, provides this high speed connectivity and is, therefore, instrumental in transforming the cable system into a full service provider of video, voice and data telecommunications services.
As mentioned above, the cable industry has been upgrading its coaxial cable systems to HFC systems that utilize fiber optics to connect head ends to fiber nodes and, in some instances, to also use them in the trunk lines of the coaxial distribution system. In way of background, optical fiber is constructed from thin strands of glass that carry signals longer distances and faster than either coaxial cable or the twisted pair copper wire used by telephone companies. Fiber optic lines allow signals to be carried much greater distances without the use of amplifiers (item 114 of FIG. 1). Amplifiers decrease a cable system's channel capacity, degrade the signal quality, and are susceptible to high maintenance costs. Thus, distribution systems that use fiber optics need fewer amplifiers to maintain better signal quality.
Digital data on the upstream and downstream channels is carried over radio frequency (RF) carrier signals. Cable modems are devices that convert digital data to a modulated RF signal and convert the RF signal back to digital form. The conversion is done at two points: at the subscriber's home by a cable modem and by a CMTS located at the head end. The CMTS converts the digital data to a modulated RF signal which is carried over the fiber and coaxial lines to the subscriber premises. The cable modem then demodulates the RF signal and feeds the digital data to a computer. On the return path, the operations are reversed. The digital data is fed to the cable modem which converts it to a modulated RF signal. Once the CMTS receives the RF signal, it demodulates it and transmits the digital data to an external source.
As mentioned above, cable modem technology is in a unique position to meet the demands of users seeking fast access to information services, the Internet and business applications, and can be used by those interested in cablecommuting (a group of workers working from home or remote sites whose numbers will grow as the cable modem infrastructure becomes increasingly prevalent). Not surprisingly, with the growing interest in receiving data over cable network systems, there has been an increased focus on performance, reliability, and improved maintenance of such systems. In sum, cable companies are in the midst of a transition from their traditional core business of entertainment video programming to a position as a full service provider of video, voice and data telecommunication services. Among the elements that have made this transition possible are technologies such as the cable modem.
FIG. 2 provides a schematic block diagram illustrating the basic components of a Cable Modem Termination System (CMTS), represented by block 200. Preferably, the CMTS is a “routing” CMTS, which handles at least some routing functions. Alternatively, the CMTS may be a “bridging” CMTS, which handles only lower-level tasks. In a specific embodiment as shown, for example, in FIG. 2, the CMTS implements three network layers, including a physical layer 232, a Medial Access Control (MAC) layer 230, and a network layer 234. When a modem sends a packet of information (e.g. data packet, voice packet, etc.) to the CMTS, the packet is received at fiber node 210 (component 108 of FIG. 1). Each fiber node 210 can generally service about 500 subscribers, depending on bandwidth. Converter 212 converts optical signals transmitted by fiber node 210 into electrical signals that can be processed by upstream demodulator and receiver 214. The upstream demodulator and receiver 214 is part of the CMTS physical layer 232. Generally, the physical layer is responsible for receiving and transmitting RF signals on the HFC cable plant. Hardware portions of the physical layer include downstream modulator and transmitter 206 and upstream demodulator and receiver 214. The physical layer also includes device driver software 286 for driving the hardware components of the physical layer.
Once an information packet is demodulated by demodulator/receiver 214, it is then passed to MAC layer 230. A primary purpose of MAC layer 230 is to coordinate channel access of multiple cable modems sharing the same cable channel. The MAC layer 230 is also responsible for encapsulating and de-encapsulating packets within a MAC header according to the DOCSIS standard for transmission of data or other information. The MAC headers include addresses to specific modems or to the CMTS (if sent upstream) by a MAC layer 230 in CMTS 200. In order for data to be transmitted effectively over a wide area network such as HFC or other broadband computer networks, a common standard for data transmission is typically adopted by network providers. A commonly used and well known standard for transmission of data or other information over HFC networks is DOSCIS. The DOCSIS standard has been publicly presented by Cable Television Laboratories, Inc. (Louisville, Colo.) in document control number SP-RFIv1.1-102-990731, Jul. 31, 1999. That document is incorporated herein by reference for all purposes.
MAC layer 230 includes a MAC hardware portion 204 and a MAC software portion 284, which function together to encapsulate information packets with the appropriate MAC address of the cable modem(s) on the system. Note that there are MAC addresses in the cable modems which encapsulates data or other information to be sent upstream with a header containing the MAC address of the CMTS associated with the particular cable modem sending the data.
In specific CMTS configurations, the hardware portions of physical layer 232 and MAC layer 230 reside on a physical line card 220 within the CMTS. The CMTS may include a plurality of distinct line cards which service particular cable modems in the network. Each line card may be configured to have its own unique hardware portions of the physical layer 232 and MAC layer 230.
Each cable modem on the system has its own MAC address. Whenever a new cable modem is installed, its address is registered with MAC layer 230. The MAC address is important for distinguishing data sent from individual cable modems to the CMTS. Since all modems on a particular channel share a common upstream path, the CMTS 200 uses the MAC address to identify and communicate with a particular modem on a selected upstream channel. Thus, data packets, regardless of format, are mapped to a particular MAC address. MAC layer 230 is also responsible for sending out polling messages as part of the link protocol between the CMTS and each of the cable modems on a particular channel. These polling messages are important for maintaining a communication connection between the CMTS and the cable modems.
After the upstream information has been processed by MAC layer 230, it is then passed to network layer 234. Network layer 234 includes switching software 282 for causing the upstream information packet to be switched to an appropriate data network interface on data network interface 202.
When a packet is received at the data network interface 202 from an external source, the switching software within network layer 234 passes the packet to MAC layer 230. MAC block 204 transmits information via a one-way communication medium to a downstream modulator and transmitter 206. Downstream modulator and transmitter 206 takes the data (or other information) in a packet structure and modulates it on the downstream carrier using, for example, QAM 64 modulation (other methods of modulation can be used such as CDMA {Code Division Multiple Access} OFDM {Orthogonal Frequency Division Multiplexing}, FSK {FREQ Shift Keying}). The return data is likewise modulated using, for example, QAM 16 or QSPK. These modulations methods are well-known in the art.
Downstream Modulator and Transmitter 206 converts the digital packets to modulated downstream RF frames, such as, for example, MPEG or ATM frames. Data from other services (e.g. television) is added at a combiner 207. Converter 208 converts the modulated RF electrical signals to optical signals that can be received and transmitted by a Fiber Node 210 to the CMTS.
It is to be noted that alternate embodiments of the CMTS (not shown) may not include network layer 234. In such embodiments, a CMTS device may include only a physical layer and a MAC layer, which are responsible for modifying a packet according to the appropriate standard for transmission of information over a cable modem network. The network layer 234 of these alternate embodiments of CMTS devices may be included, for example, as part of a conventional router for a packet-switched network.
In a specific embodiment, the network layer of the CMTS is configured as a cable line card coupled to a standard router that includes the physical layer 232 and MAC layer 230. Using this type of configuration, the CMTS is able to send and/or receive IP packets to and from the data network interface 202 using switching software block 282. The data network interface 202 is an interface component between external data sources and the cable system. The external data sources (item 100 of FIG. 1) transmit data to the data network interface 202 via, for example, optical fiber, microwave link, satellite link, or through various media. The data network interface includes hardware and software for interfacing to various networks such as, for example, Ethernet, ATM, frame relay, etc.
As shown in FIG. 2, CMTS 200 also includes a hardware block 250 which interacts with the software and other hardware portions of the various layers within the CMTS. Block 250 includes one or more processors 255 and memory 257. The memory 257 may include, for example, I/O memory (e.g. buffers), program memory, shared memory, etc. Hardware block 250 may physically reside with the other CMTS components, or may reside in a machine or other system external to the CMTS. For example, the hardware block 250 may be configured as part of a router which includes a cable line card.
Transient and Interference Noise Effecting Upstream Data Transmission
A problem common to upstream data transmission using cable systems, i.e. transmissions from the cable modem in the home back to the head end, is interference noise at the head end which lowers the signal-to-noise ratio, also referred to as carrier-to-noise ratio. Interference noise can result from numerous internal and external sources. Sources of noise internal to the cable system may include cable television network equipment, subscriber terminals (televisions, VCRs, cable modems, etc.), intermodulation signals resulting from corroded cable termini, and core connections. Significant sources of noise external to the cable system include home appliances, welding machines, automobile ignition systems, and radio broadcast, e.g. citizen band and ham radio transmissions. These ingress noise sources enter the cable system through defects in the coaxial cable line, which acts essentially as a long antenna. Ultimately, when cable systems are entirely optical fiber, ingress noise will be a far less significant problem. However, until that time, ingress noise is and will continue to be a problem with upstream transmissions.
The portion of bandwidth reserved for upstream signals is normally in the 5 to 42 MHz range. Some of this frequency band may be allocated for set-top boxes, pay-per-view, and other services provided over the cable system. Thus, a cable modem may only be entitled to some fraction (i.e., a “sub-band”) such as 1.6 MHz, within a frequency range of frequencies referred to as its “allotted band slice” of the entire upstream frequency band (5 to 42 MHz). This portion of the spectrum—from 5 to 42 MHz—is particularly subject to ingress noise and other types of interference. Thus, cable systems offering two-way data services must be designed to operate given these conditions.
As noted above, ingress noise, typically narrow band, e.g., less than 100 kHz, is a general noise pattern found in cable systems. Upstream channel noise resulting from ingress noise adversely impacts upstream data transmission by reducing data throughput and interrupting service, thereby adversely affecting performance and efficient maintenance. One strategy to deal with cable modem ingress noise is to position the modem's upstream data carrier in an ingress noise gap where ingress noise is determined to be low, such as between radio transmission bands. The goal is to position data carriers to avoid already allocated areas.
Ingress noise varies with time, but tends to accumulate over the system and gathers at the head end. In addition, while a particular frequency band may have been appropriate for upstream transmissions at the beginning of a transmission, it may later be unacceptably noisy for carrying a signal. Therefore, a cable system must attempt to identify noisy frequency bands and locate optimal or better bands for upstream transmission of data at a given time.
One method of locating an area of lower noise in an upstream path involves arbitrarily selecting frequencies from a frequency list as soon as the noise for a current frequency becomes unacceptable. The frequencies may be chosen using a round robin or other selection methodology. Another method involves deploying a spectrum analyzer to locate an appropriate frequency in a single pass. The first blind “round robin” method of picking a frequency from a frequency list (also referred to as dynamic frequency agility) is slow in locating an ingress free gap since it requires going through many frequencies before a frequency with an acceptable noise level is located. It also involves changing upstream data carrier frequencies without measuring or comparing error levels of the different frequencies before choosing a particular frequency.
Implementing the other method of using a spectrum analyzer is costly and requires another hardware component in the CMTS. It involves measuring power levels (using an FFT and FIR filter) in the entire frequency spectrum using a single sweep and identifying ingress noise gaps as power minimas at the head end. Another method utilizes a “gate” that keeps the return path from an individual subscriber closed except for those times when the subscriber actually sends a return signal upstreamn. This would require knowing when the subscriber will send a return signal or any signal upstream.
Another technique of determining whether one or more upstream receiver bands is better than the band being used involves some type of “lookahead” feature. That is, it is generally desirable to be able to see ahead and then make a decision as to which band to hop to since moving a group of cable modems from one receiver band to another continuously can result in unacceptable performance on the upstream path. Moving a group of modems to another band and testing that band results in a timing penalty and, under DOCSIS, involves having to signal the downstream receiver and MAC layer, all of which takes time. For example, suppose it takes five milliseconds for a group of modems to hop to another band and another 245 milliseconds to test that new band and determine whether it is acceptable. At this rate, it takes about one second to test only four frequencies, or 30 seconds to test 120 frequencies (not an unusually high number) continuously. However, the timeout period for many modems is 30 seconds under DOCSIS at which point the connection is lost, which can include a loss of voice calls (in cases where there is voice-over-IP) and data loss. Because the noise on the upstream is chaotic, full of slow and fast transience, it is not unusual to have to hop through hundreds or thousands of frequencies before finding an acceptable receiver band.
One way for adding a lookahead feature to a CMTS is to simply add a second physical receiver in the CMTS to act as a “lookahead” receiver. This receiver can be used to determine whether other upstream receiver bands have a better carrier-to-noise ratio (or one that is above a certain threshold). However, as with the spectrum analyzer, this solution requires an additional costly hardware component in the CMTS which is generally undesirable. Furthermore, the second “lookahead” receiver cannot perform as a normal upstream receiver since it would have to be dedicated to the lookahead function. Such a receiver is available from Arris Interactive of Atlanta, Ga.
Therefore, it would be desirable to have a lookahead feature in a cable modem plant that does not require additional hardware components in the CMTS but still has the benefit of looking ahead at other bands before hopping to those bands for a group of modems. This will result in a reliable, efficient, and cost-effective method of locating upstream receiver band in an ingress or transient noise gap, thereby enabling deliberate and intelligent placement of an upstream data carrier. Furthermore, it more fully utilizes, through software, an existing and fully functioning upstream receiver without having to add more hardware components to the CMTS or anywhere else in the cable plant.